Field of the Invention
The present invention relates to a charging device, an image forming unit including the charging device, and an image forming apparatus including the charging device; and in particular, the present invention relates to a charging device compatible with an electrophotographic method, an image forming unit including the charging device, and an image forming apparatus including the charging device.
Related Art
An image forming apparatus employing an electrophotographic method forms an image as follows. More specifically, first of all, a charging device charges a surface of a photosensitive drum. Subsequently, a laser beam, which is modulated in accordance with content of an image to be formed, is emitted onto the surface of the charged photosensitive drum, thereby reducing part of the charge on the surface of the photosensitive drum, and as a result, a latent image is formed on the photosensitive drum. A developing unit causes a toner to be attached onto the surface of the photosensitive drum, on which the latent image has been formed, thereby developing the latent image into a toner image on the surface of the photosensitive drum. Subsequently, the toner image formed on the surface of the photosensitive drum is transferred onto a recording medium such as paper, by way of a transfer unit. The image transferred onto the recording medium is fixed to the recording medium, by way of a fixing unit.
Here, part of the toner for forming a toner image will waft in the atmosphere around the periphery of the photosensitive drum, and as a result, part of the periphery of the photosensitive drum may be contaminated with such toner. In particular, a high voltage is applied to a discharging electrode provided to the charging device for charging the surface of the photosensitive drum, and the discharging electrode performs corona discharge towards the surface of the photosensitive drum. In such a circumstance, contaminant such as carbon compounds and nitrogen oxides (NOx) will attach to the discharging electrode. When such contaminant is attached to the discharging electrode in this manner, the charging device may not function sufficiently, and the photosensitive drum may be hindered from being sufficiently uniformly charged.
In line with this, inventions for removing contaminant attached to a discharging electrode have been made. For example, Patent Document 1 discloses a corona discharge device, which is provided with: a corona electrode formed in a serrated shape; a roller for removing corona products attached to the corona electrode; and a retaining portion for moving the roller along a longitudinal direction of the corona electrode; in which the corona electrode supports the roller so as to be displaceable in a direction away from the retaining portion. In addition, the corona discharge device, which is further provided with a biasing member for biasing the roller towards the corona electrode side, is also disclosed.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2010-191264
However, in a conventional scorotron charging device, for example, a discharging electrode being a corona electrode is shaped like a saw blade provided with a plurality of protruding portions. Therefore, the roller being biased towards the discharging electrode side will sequentially clean the protruding portions, from the protruding portion at one end portion to the protruding portion at the other end portion, while applying a pressure thereto. Once the roller has travelled at least a predetermined distance from the end portion of the discharging electrode, the roller maintains a height at a predetermined level or higher in a vicinity of the upper end of the plurality of protruding portions. In contrast, until the roller reaches the front end of the discharging electrode, the roller is at the level in a vicinity of the lower end of the protruding portions of the discharging electrode; therefore, the roller may directly hit the base of the protruding portion in the vicinity of the front end of the discharging electrode, and as a result, the protruding portion at the front end may be damaged.